This invention pertains to, inter alia, charged-particle-beam (CPB) optical systems as used in CPB microlithography. (Microlithography is projection-transfer of a pattern, defined by a reticle or mask, onto a sensitive substrate using an energy beam. Microlithography is a key technique used in the manufacture of microelectronic devices such as semiconductor integrated circuits, displays, and the like.) CPB optical systems typically include various CPB lenses, deflectors, and apertures. More specifically, the invention pertains to devices and methods for aligning an aperture-angle-limiting aperture with an optical axis of the CPB optical system.
Charged-particle-beam (CPB) microlithography is a candidate new-generation microlithography technique offering prospects of better image resolution than currently obtainable with optical microlithography. A CPB microlithography apparatus includes a CPB optical system and a CPB source. The CPB source produces a suitable charged particle beam, such as an electron beam or ion beam, for use as a microlithographic energy beam. The CPB optical system typically includes CPB lenses, deflectors, and apertures. One type of aperture limits the angle at which charged particles are incident on the reticle or substrate, and hence is termed herein an xe2x80x9caperture-angle-limiting aperture.xe2x80x9d
A conventional CPB optical system as used in a conventional CPB microlithography system is shown in FIG. 3. The FIG. 3 system is shown and described in the context of forming and using an electron beam as a representative charged particle beam, and in the context of employing a reticle defining a pattern that is projected onto a sensitive substrate.
The FIG. 3 system includes a source 1, an illumination-optical system IOS, and a projection-optical system POS. The source 1 produces an electron beam 3 that propagates in a downstream direction. The illumination-optical system IOS comprises components situated downstream of the source 1 and upstream of a reticle 9. The projection-optical system POS comprises components situated downstream of the reticle 9 and upstream of a sensitive substrate (or xe2x80x9cwaferxe2x80x9d) 12. By xe2x80x9csensitivexe2x80x9d is meant that the upstream-facing surface 12s of the substrate 12 is coated with a suitable material (termed a xe2x80x9cresistxe2x80x9d) that responds in an image-imprinting way to exposure by the charged particle beam. Exposure of the resist with an image of a region (e.g., a xe2x80x9csubfieldxe2x80x9d) of the reticle 9 causes xe2x80x9ctransferxe2x80x9d of an image of the respective pattern portion to the upstream-facing surface 12s. Extending through the illumination-optical system IOS and projection-optical system POS is an optical axis A.
The electron beam 3 emitted from a cathode of the source 1 forms a beam crossover 2 on the optical axis A. The beam 3 propagating downstream of the beam crossover 2 is an xe2x80x9cillumination beamxe2x80x9d that passes through a first illumination lens 4. The first illumination lens 4 forms an image of the cathode on a beam-shaping aperture 5 (defining typically a rectangular opening 5a). The beam-shaping aperture 5 trims the transverse profile of the illumination beam, according to the profile of the opening 5a, as appropriate for illuminating the desired shape and size of individual subfields or other exposure units on the reticle. Meanwhile, the first illumination lens 4 forms an image of the beam crossover 2 on an aperture-angle-limiting aperture 7. A maximal aperture angle of the beam 3 (as incident on the upstream-facing surface 12s located in the imaging plane) is imposed on the beam by the aperture-angle-limiting aperture 7.
After establishing the desired transverse dimensions of individual exposed subfields and the desired range of the aperture angle, as described above, an image of the cathode is formed on the reticle 9 by a second illumination lens 8. Portions of the illumination beam passing through a selected subfield on the reticle 9 constitute a xe2x80x9cpatterned beamxe2x80x9d that forms an image of the illuminated subfield on the upstream-facing surface 12s of the substrate (xe2x80x9cwaferxe2x80x9d) 12. Actual imaging is performed by a first projection lens 10 and a second projection lens 11 of the projection-optical system POS.
The reticle 9 defines the pattern to be exposed. In one type of conventional reticle 9 (termed a xe2x80x9cstencilxe2x80x9d reticle), openings are defined in a thin film or membrane (made of a silicon membrane or the like). The openings versus surrounding regions in the thin film define the pattern elements (i.e., the openings are transmissive to charged particles of the illumination beam and the membrane tends to block incident charged particles). In another type of conventional reticle 9, termed a xe2x80x9cscattering-membranexe2x80x9d reticle, pattern elements are defined by corresponding regions of a heavy-metal layer (that exhibits a high level of scattering of incident charged particles) situated on a CPB-transmissive membrane.
With a stencil reticle, as noted above, incident charged particles of the illumination beam not passing through an opening tend to be blocked (and absorbed) by the membrane portion of the reticle 9. This absorption causes membrane heating, especially if the membrane is thick, which results in reticle instability. Consequently, the reticle membrane usually is made sufficiently thin to transmit (with scattering) at least some of the incident charged particles. Since incident charged particles are scattered widely by such a membrane (but not by the openings in the membrane), an aperture normally is situated downstream of the reticle 9 to absorb the scattered electrons and thus prevent them from propagating to the substrate. By absorbing these scattered charged particles, appropriate contrast is obtained of the image as formed on the substrate 12.
In a conventional CPB microlithography apparatus, the center of the aperture-angle-limiting aperture 7 is located on the optical axis A. It is desirable that the propagation axis of the illumination beam be aligned with the optical axis A at the aperture-angle-limiting aperture 7. Significant misalignment causes the distribution of beam angle on the substrate to be asymmetric, which causes substantial aberration of an image as projected onto the upstream-facing surface 12s. 
To avoid or minimize Coulomb effects, a recent innovation is to configure the aperture-angle-limiting aperture 7 as an annular aperture, which produces a xe2x80x9chollowxe2x80x9d illumination beam. In this regard, reference is made to Japan Kxc3x4kai Patent Document Nos. 11-297610, filed Apr. 8, 1998, 2000-012454, filed Jun. 25, 1998, and 2000-100691, filed Sep. 21, 1998. With an annular aperture-angle-limiting aperture, misalignment of the propagation axis of the illumination beam, the optical axis A, and the center of the aperture-angle-limiting aperture 7 with each other causes marked asymmetry in the transverse distribution of beam current. Such asymmetry of beam-current density causes, in turn, a corresponding asymmetry of the Coulomb effect, making controlled reductions of the Coulomb effect especially difficult. These problems cause substantial problems with aberrations.
In FIG. 3, a deflector 6 normally is used to align the propagation axis of the illumination beam with the center of the aperture-angle-limiting aperture 7. To such end, the deflection center of the deflector 6 normally is set to the position of the beam-shaping aperture 5 to prevent the image of the beam-shaping aperture 5 from shifting laterally as the deflector 6 is energized. By energizing the deflector 6, the illumination beam is shifted laterally relative to the aperture-angle-limiting aperture 7. While energizing the deflector 6, the beam current incident to the aperture-angle-limiting aperture 7 is read using an ammeter 17. The propagation axis of the illumination beam is regarded as aligned with the center of the aperture-angle-limiting aperture 7 whenever the measured current is at a minimum, indicating completion of alignment.
Unfortunately, in the alignment method summarized above, the current reading obtained by the ammeter 17 is extremely small in any event. Consequently, the current reading at xe2x80x9calignmentxe2x80x9d can be at a level that is barely detectable. Furthermore, the ammeter 17 tends to exhibit very low sensitivity at any of various locations around the actual xe2x80x9calignedxe2x80x9d position. I.e., the ammeter 17 reads an integrated current from all locations on the aperture-angle-limiting aperture 7 at which the beam is incident, making it extremely difficult to ascertain any difference in a reading at an actual xe2x80x9calignedxe2x80x9d position versus a position characterized by substantial misalignment. As a result, it is extremely difficult to accurately determine whether the propagation axis of the beam has been aligned properly. I.e., even though it is possible to align the propagation axis of the illumination beam with the center of the aperture-angle-limiting aperture 7, it actually is extremely difficult to do so using the conventional approaches summarized above.
In view of the shortcomings of conventional methods and apparatus as summarized above, one object of the invention is to provide charged-particle-beam (CPB) optical systems, and CPB microlithography apparatus (including CPB point-beam writing apparatus and CPB projection-microlithography apparatus) comprising such optical systems, wherein the systems and apparatus exhibit reduced aberrations compared to conventional apparatus. Another object is to provide methods for accurately aligning the propagation axis of a CPB illumination beam with the center of an aperture-angle-limiting aperture.
To such ends, and according to a first aspect of the invention, methods are provided, in the context of, for example, a CPB microlithography method, for aligning the propagation axis of an imaging beam with a center of an aperture-angle-limiting aperture. In the subject CPB microlithography method, a charged-particle illumination beam, propagating from a beam source and having a respective propagation axis, is passed through an illumination-optical system that includes a lens, a deflector, and an aperture-angle-limiting aperture. In the alignment method, the following are provided: (1) a projection-optical system that forms an image of the aperture-angle-limiting aperture at an imaging plane, (2) an alignment-measurement aperture situated at the imaging plane, (3) a beam detector situated downstream of the alignment-measurement aperture, and (4) a scanning deflector, situated upstream of the alignment-measurement aperture, configured to impart a deflection in two dimensions to an imaging beam formed of a portion of the illumination beam passing through the aperture-angle-limiting aperture and having a propagation axis. The deflection is transverse to the propagation axis of the imaging beam. The scanning deflector is energized so as to cause the imaging beam to be deflected in the two dimensions over the alignment-measurement aperture, while using the beam detector to obtain an image of beam intensity of the imaging beam passing through the alignment-measurement aperture, as distributed over the two dimensions. In the image of beam intensity a point of maximum intensity, corresponding to the propagation axis of the imaging beam, is identified. Based on the two-dimensional image, the deflector in the illumination-optical system is energized as required to align the propagation axis of the imaging beam with the center of the aperture-angle-limiting aperture.
The method can include the step of determining the center of gravity of the aperture-angle-limiting aperture, wherein the center of gravity corresponds to the center of the aperture-angle-limiting aperture. The step of determining the center of gravity of the aperture-angle-limiting aperture can include the steps of: (1) converting the two-dimensional image to a binary image of the aperture-angle-limiting aperture, and (2) from the binary image, determining the center of gravity of the aperture-angle-limiting aperture.
The method can include the step of smoothing the two-dimensional intensity distribution, wherein the point of maximum intensity is identified based on the smoothed distribution.
According to another aspect of the invention, CPB optical systems are provided. An embodiment of such a system comprises an illumination-optical system situated along an optical axis and including an illumination lens, a deflector, and an aperture-angle-limiting aperture. The illumination-optical system is transmissive to an illumination beam propagating from a beam source. The system also comprises a projection-optical system situated along the optical axis downstream of the illumination-optical system. The projection-optical system includes a projection lens and is transmissive to an imaging beam propagating along a propagation axis from the illumination-optical system. The system also includes a beam-aligmnent device configured to determine a condition of alignment of the propagation axis of the imaging beam with a center of the aperture-angle-limiting aperture. The beam-alignment device comprises: (1) an alignment-measurement aperture situated at an imaging plane; (2) a beam detector situated downstream of the alignment-measurement aperture; (3) a scanning deflector situated upstream of the alignment-measurement aperture, the scanning deflector being configured to deflect the imaging beam in two dimensions, perpendicular to the optical axis, over the alignment-measurement aperture; and (4) a controller connected to the beam detector and the scanning deflector. The controller is configured to (a) energize the scanning deflector so as to deflect the imaging beam (carrying an image of the aperture-angle-limiting aperture) in the two dimensions over the alignment-measurement aperture on the imaging plane, (b) obtain a signal from the beam detector corresponding to a two-dimensional image of intensity of the imaging beam passing through the alignment-measurement aperture, (c) identify a point of maximum intensity corresponding to the propagation axis, and (d) based on the two-dimensional image, energize the deflector in the illumination-optical system as required to align the propagation axis with the center of the aperture-angle-limiting aperture.
According to another aspect of the invention, CPB microlithography apparatus are provided that comprise a CPB optical system as summarized above. The CPB microlithography apparatus are not limited to xe2x80x9cprojection-microlithographyxe2x80x9d apparatus. Also encompassed are any of various other types of CPB microlithography apparatus, such as CPB point-beam writing apparatus.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.